Search Images Maps Play YouTube News Gmail Drive More »
Sign in
Screen reader users: click this link for accessible mode. Accessible mode has the same essential features but works better with your reader.

Patents

  1. Advanced Patent Search
Publication numberUS5391215 A
Publication typeGrant
Application numberUS 07/923,456
Publication dateFeb 21, 1995
Filing dateAug 3, 1992
Priority dateAug 3, 1992
Fee statusLapsed
Also published asEP0582006A1, EP0582006B1
Publication number07923456, 923456, US 5391215 A, US 5391215A, US-A-5391215, US5391215 A, US5391215A
InventorsKenichi Kobayashi, Tatsuhiko Fujinuma
Original AssigneeJapan Metals & Chemicals Co., Ltd.
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Method for producing high-purity metallic chromium
US 5391215 A
Abstract
There is proposed a novel method for manufacturing high-purity metallic chromium that can eliminate the problems of reduced heating capability of the furnace, contamination of produced metallic chromium and other disadvantages related to the operation of the furnace. According to the invention, one or more than one of easily sulfidable metals selected from Sn, Ni and Cu are added to crude metallic chromium containing impurities and the mixture is loaded into a vacuum furnace equipped with heating elements of graphite and heated to 1,200 to 1,500 C. in an atmosphere with reduced pressure of between 0.1 and 5 torr.
Images(3)
Previous page
Next page
Claims(10)
What is claimed is;
1. A method for manufacturing high-purity metallic chromium having reduced levels of sulfur and nitrogen, comprising steps of mixing powdered crude metallic chromium containing impurities with powder of one or more than one easily sulfidable metals selected from Sn, Ni and Cu and subjecting the mixture to a heat treatment process in vacuum, said heat treatment process being conducted at a temperature between 1,200 and 1,500 C. and pressure between 0.1 and 5 torr in a vacuum furnace equipped with heating elements of graphite.
2. A method for manufacturing high-purity metallic chromium according to claim 1, wherein a binding agent is added to said mixture to form briquettes of the mixture, said briquettes being subsequently subjected to a heat treatment process.
3. A method for manufacturing high-purity metallic chromium according to claim 1, wherein the easily sulfidable metals are added in said mixture in an amount such that a ratio of said amount to a stoichiometric amount of easily sulfidable metals for reacting with the sulfur in the crude metallic chromium to metal sulfide is between 0.9 and 1.1.
4. A method of manufacturing high-purity metallic chromium according to claim 1, wherein the high-purity metallic chromium contains less than 10 ppm of sulfur, less than 10 ppm of nitrogen, and about 200 ppm of oxygen.
5. A method for manufacturing high-purity metallic chromium having reduced concentration levels of sulfur, nitrogen and oxygen, comprising steps of mixing powdered crude metallic chromium containing impurities with carbon powder and powder of one or more than one easily sulfidable metals selected from tin, nickel and copper and subjecting the mixture to a heat treatment process in vacuum, said heat treatment process being conducted at a temperature between 1,200 and 1,500 C. and pressure between 0.1 and 5 torr in a vacuum furnace equipped with heating elements of graphite.
6. A method for manufacturing high-purity metallic chromium according to claim 5, wherein a binding agent is added to said mixture to form briquettes of the mixture, said briquettes being subsequently subjected to a heat treatment process.
7. A method for manufacturing high-purity metallic chromium according to claim 5, wherein the easily sulfidable metals are added in said mixture in an amount such that a ratio of said amount to a stoichiometric amount of easily sulfidable metals for reacting with the sulfur in the crude metallic chromium to metal sulfide is between 0.9 and 1.1.
8. A method for manufacturing high-purity metallic chromium according to claim 5, wherein the carbon powder is added to said mixture in an amount such that a ratio of said amount of carbon powder to a stoichiometric amount of carbon for reacting with the oxygen in the crude metallic chromium to carbon monoxide is between 0.9 and 1.1.
9. A method for manufacturing high-purity metallic chromium according to claim 5, wherein the high-purity metallic chromium contains less than 10 ppm of sulfur, less than 10 ppm of nitrogen, and about 200 ppm of oxygen.
10. A method for manufacturing high-purity metallic chromium having reduced levels of sulfur, nitrogen and oxygen, comprising steps of mixing powdered crude metallic chromium containing impurities with chromium carbide powder and powder of one or more than one easily sulfidable metals selected from Sn, Ni and Cu and subjecting the mixture to a heat treatment process in vacuum, said heat treatment process being conducted at a temperature between 1,200 and 1,500 C. and pressure between 0.1 and 5 torr in a vacuum furnace equipped with heating elements of graphite.
Description
BACKGROUND OF THE INVENTION

This invention relates to a method for producing high-purity metallic chromium and, particularly, it relates to a method for producing metallic chromium with a very low concentration level of impurities such as sulfur, nitrogen and oxygen. Such high-purity metallic chromium can be suitably used as a raw material for the electronic industry as well as for the industry of producing corrosion-resistive and heat-resistive alloys (super alloys).

Known methods for producing metallic chromium include the electrolytic method that decomposes Cr2 (SO4)3 by electricity and the alumino-thermite reduction method that reduces Cr2 O3. However, metallic chromium obtained by any of these known methods contains S, O and N at a relatively high level and, therefore, is not good for manufacturing electronic products.

More specifically, said electrolytic method uses Cr2 (SO4)3 as electrolyte and, therefore, the resultant metallic chromium contains S at a relatively high level of concentration between 200 and 300 ppm and contains O at a level between 3,000 and 10,000 ppm and N between 200 and 500 ppm because of the use of aqueous electrolyte.

On the other hand, metallic chromium obtained by the thermite reduction method contains S at a level of concentration as high as between 200 and 400 ppm because of the fact that sulfuric acid is used for production of Cr2 O3 to be used as the source material and that almost all the sulfur contained in the source material remains in the resultant metallic chromium. While the O content can be decreased by increasing the rate of reducing agent (aluminum) to be added to the source material, this in turn causes the aluminum to remain in the resultant metallic chromium at a high concentration level. If the rate of the use of aluminum should be reduced, the O concentration level of the obtained metallic chromium becomes inevitably as high as 1,000 to 4,000 ppm. The N concentration level will also be as high as approximately 200 ppm.

Since metallic chromium produced by any of the known methods contains S, O and N at a relatively high concentration level, these impurities should be thoroughly removed from the metallic chromium if it be suitably used for its applications.

The vacuum carbon reduction method and the hydrogen reduction method are among the known methods for degassing metallic chromium. With the vacuum carbon reduction method, carbon powder is added to powdered crude metallic chromium and the mixture is then heated in vacuum to remove the oxygen contained in the metallic chromium after turning it into CO. The hydrogen reduction method is, on the other hand, a method of degassing metallic chromium by heating powdered metallic chromium in an atmosphere of hydrogen and causing the oxygen contained in it to change to H2 O.

However, any of the above described known methods cannot meet the requirement of manufacturing high-purity metallic chromium which is needed for highly advanced electronic products.

In view of these circumstances, one of the inventors of the present invention has proposed a method for manufacturing high-purity metallic chromium with a very low concentration level of impurities such as S, O and N as disclosed in Japanese Patent Publication No. 3-79412. The proposed method in fact consists in combining a method of heating in vacuum powder of crude metallic chromium with that of easily sulfidable metals such as Sn, Ni and Cu and the vacuum carbon reduction method or the hydrogen reduction method as described above.

It has been proved that the proposed method is very effective in manufacturing high-purity metallic chromium with a very low concentration level of impurities and, therefore, can be suitably used for various applications including those described above.

However, since the proposed method requires a high degree of vacuum and elevated temperature for heat treatment of crude metallic chromium in vacuum, it inevitably entails a problem of sublimated metallic chromium, which eventually adheres to the heating elements and the lining of furnace to damage the furnace and reduce its heat treatment capacity so that consequently the capability of the furnace to produce high-purity metallic chromium on a stable basis may be significantly adversely affected. There may also arise a problem of contamination of produced metallic chromium by the metallic material of the heating elements of furnace if the heating elements are made of metal. Additionally, there may also be a problem of malfunction of furnace due to prolonged furnace operation involving vacuum and high temperature in an attempt to reduce the concentration level of impurities in the produced metallic chromium as low as possible.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a method as well as apparatus for manufacturing high-purity metallic chromium that can solve the above described problems of deteriorated heat treatment capacity, production of contaminated metallic chromium and furnace malfunction.

As a result of intensive research efforts, the inventors of the present invention have proposed a method for manufacturing high-purity metallic chromium which is free from the above described problems.

According to the present invention, there is provided a method for manufacturing high-purity metallic chromium comprising steps of mixing powdered metallic chromium containing impurities with powder of one or more than one easily sulfidable metals selected from Sn, Ni and Cu and subjecting the mixture to a heat treatment process in vacuum, said heat treatment process being conducted at a temperature between 1,200 and 1,500 C. and pressure between 0.1 and 5 torr in a vacuum furnace equipped with heating elements of graphite.

For the purpose of the present invention, carbon powder may be advantageously added to said mixture.

A binding agent may be advantageously added to said mixture to form briquettes of the mixture, which are then subjected to a heat treatment process.

For the purpose of the present invention, the volume of carbon powder to be added to said briquetted mixture needs to be such that the ratio of said volume of carbon powder to the stoichiometric volume of carbon for reducing the oxygen in the crude metallic chromium is found between 0.9 and 1.1. On the other hand, the volume of powder of the easily sulfidable metals in said mixture is preferably such that the ratio of said volume to the stoichiometric volume of easily sulfidable metals for removing the sulfur in the crude metallic chromium is also found between 0.9 and 1.1.

According to the present invention, there is also provided an apparatus for manufacturing high-purity metallic chromium comprising a container made of graphite for containing a mixture of powdered metallic chromium, easily sulfidable metals and carbon powder, a thermally insulating box provided in its inside with heating elements made of graphite and a lining made of carbon for receiving said container and a vacuum furnace made of steel and provided with a lid for sealingly containing said thermally insulating box and said graphite container.

With a method and an apparatus for manufacturing high-purity metallic chromium according to the invention, high-purity metallic chromium which is free from impurities such as S, O and N that inevitably contaminate refined metallic chromium if an ordinary method is used can be produced in an effective and efficient manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the effect of Sn added to crude metallic chromium for removing S in the latter.

FIG. 2 is a graph showing the effect of C added to crude metallic chromium for removing O in the latter.

FIG. 3 is a graph showing the effect of duration of heat treatment of crude metallic chromium for removing S and O in the latter.

FIG. 4 shows two sectional views of an embodiment of the apparatus for manufacturing high-purity metallic chromium according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Crude metallic chromium which is the starting raw material to be treated for the purpose of the invention may be prepared by means of an electrolytic method, an alumino-thermite method or a carbon reduction method. The prepared crude metallic chromium is preferably crushed to particles of 100 mesh or less in order to provide a good contact between the impurities contained in the crude metallic chromium and the additive to be added to the crude metallic chromium and clean the crude chromium as neatly as possible.

For the purpose of the present invention, powder of at least one of easily sulfidable metals selected from Sn, Ni and Cu may be advantageously added with carbon powder to powdered crude metallic chromium to form a mixture thereof.

Powder of one or more than one easily sulfidable metals is added to crude metallic chromium in order to remove the sulfur content of the crude metallic chromium. These metals easily react with sulfur to produce sulfides of the metals, which can be easily volatilized and removed when heated under reduced pressure because of its relatively high specific vapor pressure.

The volume of powder of easily sulfidable metals to be added to crude metallic chromium is preferably such that the ratio of said volume to the stoichiometric volume of easily sulfidable metals for removing the sulfur in the crude metallic chromium is found between 0.9 and 1.1. The reason for this is that, if the ratio is smaller than 0.9, the sulfur in the crude metallic chromium will be poorly removed whereas, if the ratio is greater than 1.1, the residual easily sulfidable metals in the crude metallic chromium will be significant after removing the sulfur content so that the purity of the refined metallic chromium product will be rather poor. The graph of FIG. 1 shows the effect of Sn added to crude metallic chromium for removing S in the latter and it will be seen from the graph that S is effectively removed if the ratio of Sn/S is found within the above defined range.

Carbon powder to be used with or in place of easily sulfidable metals for removing a relatively small amount of oxygen contained in crude metallic chromium for the purpose of the present invention may be replaced by chromium carbide as proposed earlier by the inventors of the present invention. (See Japanese Patent Laid-Open Publication No. 4-160124). The reason for using carbon is that oxygen in crude metallic chromium can be turned to CO gas through reaction of oxygen in crude metallic chromium and carbon powder if the mixture of crude metallic chromium and carbon powder is heated under reduced pressure and the produced CO gas can be removed by dissipating it from the reaction system. The volume of carbon powder to be added to said briquetted mixture needs to be such that the ratio of said volume of carbon powder to the stoichiometric volume of carbon for reducing the oxygen in the crude metallic chromium is found between 0.9 and 1.1. The reason for this is that, if the ratio is smaller than 0.9, the oxygen in the crude metallic chromium will be poorly removed whereas, if the ratio is greater than 1.1, the residual carbon powder in the crude metallic chromium will be significant after removing the oxygen content so that the purity of the refined metallic chromium product will be rather poor. This will also be understood from the graph of FIG. 2.

For the purpose of the present invention, said mixture is heated under reduced pressure. Said mixture may be heated as it is or, alternatively, it may be molded after adding a binding agent thereto. Possible modes of molding may include briquetting and pelletizing. While no specific requirements need to be defined for molded pieces of crude metallic chromium in terms of shape and size, each molded piece of crude metallic chromium may preferably have a form that permits easy handling for subsequent operations. While water may be used as a binding agent to be used for the purpose of the invention, an organic binding agent such as polyvinyl alcohol can be more advantageously used.

When the powder is molded into briquettes by using a binder agent, they are preferably dried at a temperature that does not cause oxidization of metallic chromium prior to the process of depressurization and heat-treatment.

For the above described heat treatment to be conducted for the purpose of the present invention, a vacuum furnace as illustrated in (a) and (b) of FIG. 4 will be used. The vacuum furnace principally comprises a container 1 made of graphite, a thermally insulating box 2 that surrounds the container 1 and a vacuum furnace 3 provided with a lid for containing said thermally insulating box 2.

Said powdered or molded mixture 6 is placed in said graphite container 1. Said thermally insulating box 2 is equipped with a number of heating elements 4 made of graphite which are disposed within said box 2 and provided with a lining 5 made of carbon. Said vacuum furnace 3 is made of steel and provided with a lid 3a for sealingly enclosing the contents.

The reason for using graphite-made heating elements 4 disposed within said box 2 is that, if heating elements that are made of a metal, an oxide or a non-metal material such as SiC are used, vapor of chromium volatilized from metallic chromium during the heat treatment process in vacuum can be deposited on the heating elements to damage and degrade them until they become non-operational for prolonged or repetitive use and also the produced metallic chromium is contaminated by the vaporized component from heating elements.

If, on the other hand, such heating elements are used with low temperature and a reduced degree of vacuum in order to avoid the above problems, the time required for the overall reaction will be significantly prolonged. On the contrary, heating elements made of graphite are free from the problems of degradation due to vapor deposition, volatilization of the material of the heating elements and, therefore, contamination of the produced metallic chromium.

The above described heat treatment process is conducted in vacuum by loading a mixture of powdered crude metallic chromium, powder of one or more than one of easily sulfidable metals selected from Sn, Ni and Cu and carbon powder or briquettes thereof into said graphite container 1, placing said graphite container 1 in the thermally insulating box 2 equipped with graphite heating elements 4, closing the lid 3a of the vacuum furnace 3 and heating the mixture under reduced pressure.

The temperature and the pressure of the heat treatment needs to be respectively between 1,200 and 1,500 C. and between 0.1 and 5 torr. The reaction proceeds too slow and insufficient desulfurization and deoxidization of the reaction system will result if the temperature is below 1,200 C. On the other hand, the loss of chromium will become remarkable due to volatilization if the temperature is above 1,500 C. The loss of chromium will also be remarkable due to volatilization if the pressure is below 0.1 torr, whereas insufficient desulfurization and deoxidization will take place if the pressure is above 5 torr.

While the reaction may proceed considerably well under reduced pressure regardless of the type of atmosphere, it will be carried out more satisfactorily if it is conducted in an atmosphere of inert gas having a reduced pressure because the inert gas acts as carrier gas that enhances the mobility of the gas generated in the reaction system by heat treatment.

While the duration of the heat treatment with the above described temperature range cannot be specifically defined because it is a function of certain variables including the volume of easily sulfidable metals, that of carbon powder and the pressure and temperature of the reaction system, 6 to 10 hours will be reasonable, although the reaction terminates an active phase in approximately 2 hours as typically illustrated in FIG. 3. As a matter of course, the heat treatment can be maintained for a more prolonged period of time and the volume of O and S will be reduced gradually in proportion to the actual duration of heat treatment.

EXAMPLE 1

Crude metallic chromium was crushed to particles of 100 mesh or less by means of a top grinder and powdered Sn and C were added to and mixed with the obtained powder of crude metallic chromium. The volume of Sn powder was so determined that its ratio to the stoichiometric volume of Sn required to change the entire S contained in the crude metallic chromium to SnS was 1.04. Similarly, the volume of C powder was so determined that its ratio to the stoichiometric volume of C required to change the entire O contained in the crude metallic chromium to CO was 1.04.

A small amount of PVA (5%) solution was added to the mixture as a binder agent and the mixture was then briquetted and dried at 130 C. for approximately 8 hours.

The obtained briquettes were then loaded into a box-shaped graphite container, which was then placed in a vacuum furnace provided in the inside with heating elements of graphite and having a thermally insulating box in it, said box being lined by a sheet of graphite. The lid of the furnace was hermetically closed and the inside of the furnace was evacuated. Thereafter, the furnace was heated while maintaining the evacuated condition of the inside to approximately 2 torr and causing argon gas to incessantly circulate there. As soon as the inside of the furnace reached a predetermined temperature, the inside pressure was gradually reduced until it finally became equal to 0.1 torr.

The argon gas was made to circulate well after the end of the heat treatment until the temperature fell below 200 C. After the inside of the furnace was sufficiently cooled, the reaction product was taken out of the container and subjected to a chemical analysis. Thereafter, a number of similar experiments and analytic operations were conducted. Table 1 shows the results of the experiments in terms of the concentration levels of impurities contained in the crude metallic chromium, the conditions of heat treatment and the concentration levels of impurities contained in the refined metallic chromium.

                                  TABLE 1__________________________________________________________________________    crude metalic  treatment                     refined metallicNo. of    chromium (ppm) conditions                     chromium (ppm)exp.    C  S  O  N  Sn temp.                  time                     C  S  O  N  Sn__________________________________________________________________________1   115  234     6100         45           <1 1340                  4  120                        28 350                              <10                                 112   "  "  "  "  "  "   6  80 12 240                              <10                                 53   "  "  "  "  "  "   8  50 7  180                              <10                                 74   "  "  "  "  "  "   10 50 5  170                              <10                                 95   "  "  "  "  "  "   14 30 4  100                              <10                                 56   "  "  "  "  "  "   20 20 5  120                              <10                                 57   103  227     5400        430           <1 1250                  6  70 15 270                               60                                 78   "  "  "  "  "  1350                  6  50 8  250                               20                                 79   "  "  "  "  "  1450                  6  50 10 210                               30                                 910  125  187     6200        307           <1 1350                  6  60 7  250                               10                                 511  "  "  "  "  "  "   6  80 7  270                               20                                 612  "  "  "  "  "  "   6  80 6  280                               20                                 5__________________________________________________________________________ [Note]- Unit; temp. = C., time = hours
EXAMPLE 2

Briquettes containing mainly crude metallic chromium and prepared in a manner similar as those of Example 1 above were subjected to a series of heat treatments conducted at 1,350 C. for 30 times, each lasted in average for 8 hours. It was found after the experiment that the vacuum furnace used for the experiment was totally free from damage and could be used for continuous operations. It was also found that the obtained refined metallic chromium was highly pure and contained O, S and N to respective concentration levels of approximately 200 ppm. less than 10 ppm and less than 10 ppm.

EFFECTS

As is apparent from the above description, a method for manufacturing high-purity metallic chromium according to the invention is advantageous in that the produced metallic chromium is free from contamination and it does not involve any reduction in the capacity of refining crude metallic chromium and the service life of vacuum furnace so that it can produce high-purity metallic chromium effectively and efficiently.

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US2834671 *May 13, 1954May 13, 1958Gordon Poole HenryMethod of producing molybdenum
US2850378 *Dec 17, 1956Sep 2, 1958Walter M WeilProduction of chromium by low-pressure reduction of oxides
US2939784 *Sep 26, 1958Jun 7, 1960Union Carbide CorpProcess for elimination of sulfur from metals
US3502461 *Dec 10, 1965Mar 24, 1970Elektrometallurgie GmbhMethod of reducing oxidic raw materials
US3794482 *Feb 5, 1971Feb 26, 1974Parlee Anderson CorpCarbothermic reduction method for converting metal oxides to metal form
US3945863 *Sep 20, 1973Mar 23, 1976Martin Marietta CorporationProcess for treating metal powders
US4662939 *Feb 21, 1986May 5, 1987Pfizer Inc.Process and composition for improved corrosion resistance
US4722826 *Sep 15, 1986Feb 2, 1988Inco Alloys International, Inc.Production of water atomized powder metallurgy products
US5092921 *Oct 25, 1990Mar 3, 1992Japan Metals & Chemicals Co., Ltd.Method for producing high-purity metallic chromium
JPH0379412A * Title not available
JPH04160124A * Title not available
Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US5561833 *Nov 21, 1994Oct 1, 1996Japan Metals & Chemicals Co., Ltd.Method of making high oxygen chromium target
US8440112 *Oct 2, 2009May 14, 2013Meiden T&D CorporationElectrode material for vacuum circuit breaker and method of manufacturing the same
US20090162273 *Dec 21, 2007Jun 25, 2009Howmedica Osteonics Corp.Chromium oxide powder having a reduced level of hexavalent chromium and a method of making the powder
US20110204299 *Oct 2, 2009Aug 25, 2011Japan Ae Power Systems CorporationElectrode material for vacuum circuit breaker and method of manufacturing the same
CN102171780BOct 2, 2009Dec 11, 2013株式会社明电舍Electrode material for vacuum circuit breaker and method for producing same
Classifications
U.S. Classification75/623
International ClassificationC22B34/32, F27B5/04, C22B1/242
Cooperative ClassificationF27B5/04, C22B34/32, C22B1/242
European ClassificationC22B1/242, C22B34/32, F27B5/04
Legal Events
DateCodeEventDescription
Aug 3, 1992ASAssignment
Owner name: JAPAN METALS & CHEMICALS CO., LTD., JAPAN
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST.;ASSIGNORS:KOBAYASHI, KENICHI;FUJINUMA, TATSUHIKO;REEL/FRAME:006230/0749
Effective date: 19920728
Sep 15, 1998REMIMaintenance fee reminder mailed
Feb 22, 1999FPAYFee payment
Year of fee payment: 4
Feb 22, 1999SULPSurcharge for late payment
Feb 21, 2003LAPSLapse for failure to pay maintenance fees
Apr 22, 2003FPExpired due to failure to pay maintenance fee
Effective date: 20030221